Dry type transformers Zaragoza Traction application © ABB Group 2009 | Slide 1

Index The product Traction transformers Special transformers Overloads Effects of harmonics on the transformer Special design © ABB Group 2009 | Slide 2

The product From 250 kVA up to 40 MVA. High voltage: up to 72.5 kV. Classes: E2, C2, F1. Partial discharges: <10 pC. Insulation class F or H. Overloads from duty class. 6, 12 pulses, or pseudo 24 pulses. Shift phase (+7,5º +15º …). © ABB Group 2009 | Slide 3

The product Vacuum cast coil dry type transformer © ABB Group 2009 | Slide 4

Traction transformers I Transformers used in variable speed drives that will feed traction systems, such as systems in: Railway application. Undergrounds. Tramways. © ABB Group 2009 | Slide 5

Traction transformers II Applications AC Transformers which feed systems in alternative current. DC Transformers which feed systems in direct current. © ABB Group 2009 | Slide 6

Special transformers 1. Overloads from duty demand. Transformers for traction applications are non - standard transformers due to: 1. Overloads from duty demand. 2. Harmonics from rectifiers. © ABB Group 2009 | Slide 7

Overloads According to EN 50329. Each duty class correlates with an overload cycle. © ABB Group 2009 | Slide 8

Overload cycles I © ABB Group 2009 | Slide 9

Duty class according to UNE EN 50329 Example for a dutty class VI (rated power in kVA) and winding temperature rise admissible. Rated power SN: 1000 kVA. (1) Each power at each overloading condition, is referred to the fundamental component of the rated power. Heating temperature test must be performed at the rated power (including the current harmonics). (2) According to the IEC 60905 winding temperature rise during the overload, must not exceed 120 K. Power Heating p.u of Ib p.u of In Duration kVA (K) 1.215 1   2400 100 a 0.823 Cont 1975.2 80 b 1.5 1.234 2h 2961.6 120 c 3 2.468 60s 5923.2 © ABB Group 2009 | Slide 10

Effects of harmonics on transformers I Harmonics are distortions of the mains supply occurring at multiples of the supply frequency. Any equipment which uses electronics to change one voltage and / or frequency to another will generate harmonic currents and consequently voltage distortion. © ABB Group 2009 | Slide 11

Effects of harmonics on transformers II Source of current harmonics Switching the line current with line frequency or its multiple by means of electronic switches. No-linear impedance. Current dependant resistances: Arc furnaces, welding machines, fluorescent lamps… Voltage dependant inductance: Transformers and core reactors. Switching on saturable inductance as induction motors or transformers. Non linear load generate harmonics © ABB Group 2009 | Slide 12

Effects of harmonics on transformers III Sources of current harmonics. Rectifiers bridges feed. Sources of voltage harmonics. Voltage drops in circuit impedance due to current harmonics. Voltage shape not fully sinusoidal. Effects of voltage harmonics. Increase of no load losses. Increase of noise level. Effects of current harmonics. Increase of the load losses. Local overheating due to uneven distribution of the eddy losses. Eventually resonance over voltages. © ABB Group 2009 | Slide 13

Effects of harmonics of transformers IV Evaluation of harmonic content Total harmonic distortion factor: The ratio of the r.m.s. value of the sum of all the harmonic components up to a specific order and the r.m.s. value of the fundamental component: The limitation of THD is aimed to prevent the simultaneous presence of several harmonics components with high amplitude. © ABB Group 2009 | Slide 14

Effects of harmonics of transformers V Compatibility levels for voltage harmonics. According to IEC 61000-2-4 for class 3 environments: THD  10% There are also limitations for individual value: Odd order excluding multiple of three (3,9,15…). Even order. Odd order multiple of three. Inter harmonics. © ABB Group 2009 | Slide 15

Special design I Due to the flow of harmonic currents in both low voltage and high voltage windings, there are extra losses and extra heating, thus the transformer must be over rated according to a higher equivalent power. Due to the flow of harmonic currents through the network and the transformer impedance, there is a voltage distortion (voltage harmonics) on the transformer magnetic core, which could saturate it. In order to avoid core saturation, the magnetic core must be over sized. © ABB Group 2009 | Slide 16

Special design II To avoid capacitate coupling between high voltage and low voltage and protect the power electronics devices on low voltage side from over voltages in high voltage side, it is recommended to place an electrostatic shield between high voltage and low voltage windings. In some cases due to floating systems or high du/dt, higher insulation levels on low voltage are needed. © ABB Group 2009 | Slide 17

Special design III The number of turns of the two low voltage windings must be modified in order to reach the voltage ratio between these two low voltage windings. The impedance between the two low voltage windings must be matched by calculating and manufacturing carefully the winding dimensions. © ABB Group 2009 | Slide 18

Special design IV The space factor is more critical because of the insulation gap between windings, and the larger size of the transformer. In order to guarantee the correct losses and good operation, the HV winding is split in two or more parallel circuits with two o more tap changers instead of one circuit. © ABB Group 2009 | Slide 19

© ABB Group 2009 | Slide 20